The present state-of-the-art in contact lenses involves two broad categories of materials: hard or rigid lenses, and soft lenses. Hard lenses were originally made of PMMA(polymethyl metacrylate), but those in widest use now are silicon/acrylates. Silicone/acrylates contain different component ratios (silicone: acrylate) or other additives such as fluorine to make them unique among a large group of related compounds. Soft lenses are generally made of polyhydroxymethylmethacrylate, crosslinked with a variety of compounds to create a lens with distinctive properties. There are other materials and configurations available (and many unique materials not presently available for clinical use), but the foregoing compositions are in widest use.
Although the present invention will primarily be discussed with reference to soft contact lenses, it is equally applicable to hard or other contact lenses.
Soft lenses are extremely hydrophilic by nature, absorbing about 25%-80% of their composition as water. Accordingly, they provide an excellent medium for microbial growth and concentration of contaminants both inside the lens, and on the surface, these microbes and contaminants typically come from the eye, the patient's hands or the solution in which the lenses are stored. As a result, the lenses must be regularly cleaned and disinfected upon removal, but not usually more than once a day.
Cleaning is generally accomplished using a surfactant which is rubbed on the lens using the fingers. This is rinsed off using saline (isotonic NaCl solution) solution. The lenses are then ready to be disinfected.
There are presently three broad categories of disinfection methods. All leave the lenses nearly isotonic to human tears, and at a substantially neutral pH (in order to decrease the potential for irritation upon reinsertion of the lenses on the eye).
The first method uses heat. The lenses are placed in a container with saline, followed by insertion of the container into a heating unit. The temperature in the heating unit is the raised to 80 to 100 degrees centrigrade for 30 minutes to one hour. Thereafter the unit automatically shuts off, and is allowed to cool for an equal length of time before the lenses can be placed on the eyes. One drawback of heat disinfection is that it cannot be used on lenses which are greater than 50% water, or damage to the lens will result. There is also evidence that lenses which are less than 50% water are prematurely aged by the heating process. Furthermore, small amounts of debris and organic material remain on the lenses even after cleaning. These literally become baked on the lenses by the sterilizing heat.
The second disinfection method uses a microbicidal chemical in saline solution. Lenses are placed in the solution for about four hours, rinsed with saline, and reinserted on the eyes. Exemplary microbicides include chlorhexidine, benzalkonium chloride, and polyaminopropyl biguanide. Although this method is a simple and generally effective way to disinfect lenses, many patients are sensitive to the microbicides used above, and cannot use them. Generally recognized sensitivity rates are between 10 to 40% of the general population. "New" Chemical sterilants simply cause the same allergy and sensitivity problems shortly after appearing on the market.
The third and most popular disinfection method uses 3% hydrogen peroxide (H.sub.2 O.sub.2) to sterilize the lenses, followed by neutralization of the hydrogen peroxide to simple biocompatible products. Neutralization must occur since the eyes can be damaged by hydrogen peroxide, and even 50 ppm H.sub.2 O.sub.2 may cause mild irritation. A one-step H.sub.2 O.sub.2 disinfection process involves placing the lenses in a container of hydrogen peroxide and saline which has a platinum catalyst at the bottom to neutralize the H.sub.2 O.sub.2 while disinfecting the lenses (U.S. Pat. No. 3,912,451). Six hours are required for this process. However, a major disadvantage of this process is that the H.sub.2 O.sub.2 is neutralized logarithmically (i.e. the concentration of H.sub.2 O.sub.2 is reduced quite rapidly in the beginning). As a result, effective microbicidal concentrations may not be present long enough to do a good job of disinfection. Even under optimum conditions, H.sub.2 O.sub.2 is only a fair bactericide, and is not a good sporicide or fungicide.
Several other systems using H.sub.2 O.sub.2 and a variety of neutralizers (or dilution with saline) involve two or more steps. For example, the lenses are placed in 3% H.sub.2 O.sub.2 for 5-20 minutes, followed with one or more rinses, and placement in a neutralizer and isotonic saline for 10 minutes to one hour. Drawbacks of this process typically involve a lack of patient compliance or inadequate removal of H.sub.2 O.sub.2. Some patients forget to complete the second neutralizing step described above, with resultant eye damage, or don't leave the lenses in the H.sub.2 O.sub.2 long enough for effective disinfection to occur. An additional problem is that many patients do not disinfect their lenses because of the perceived inconvenience of having to complete two steps. Single step systems have a substantially greater compliance rate.
A further problem has recently developed involving the appearance of new microbes and their effect on contact lens users. Two of these microbes includes the AIDS virus and Acanthamoeba. The AIDS virus is a well known, potentially lethal virus which has been isolated in tears. The importance of killing this virus is more pronounced in the doctor's office, where the same lens can be used on many different patients. Present studies indicate that the AIDS virus can be killed by either heat or H.sub.2 O.sub.2. However, Acanthamoeba is capable of spore formation, and is resistant to H.sub.2 O.sub.2. However, heat is fairly effective against Acanthamoeba. Acanthamoeba, although rare, causes a particularly painful eye infection, with no acceptable cure at present. It usually results in permanent blindness to that eye. Accordingly, many eye doctors are turning back to heat disinfection, although this method cannot be used with contact lenses having a 50% or greater water content. Furthermore, heat does not kill Bacillus organisms, as discussed in Shovlin J. "Contact Lens Q and A." Review of Optometry 1988 (March 15); 125(3): 95.
Thus the three major problems (as seen by today's contact lens practitioner) with existing state-of-the-art contact lens disinfection systems are: